If you were to change only the fronts, you'll need to match the existing wiring polarity on the rears. I forgot where I saw it, but there has been at least one incorrect post of polarity - use a 1.5v battery connected to the speaker to check, the cone moves out the front of the speaker for positive polarity.

See the photos in my posts for matching Infinity polarity to the stock speakers (felt tip pen marks I added) - all four factory speakers are the same.

If you get the polarity reversed on one speaker, and correct on the other, you will see greatly diminished bass. To check, simply use the balance/fader to select one speaker at a time and listen to music with a lot of low bass. If the bass gets louder with one speaker selected, you have polarity problems.

There is no easy way to meter the polarity from the head unit. Simply look at the OEM connectors and figure it out as described above.

I'm still not sure why all this guessing is necessary? If you solder every speaker the same, using the same plug/connector configuration as stock, how can the polarity be wrong? Surely Nissan wouldn't wire them wrong.